The synthesis of glycerol carbonate was described for years. A reaction of glycerol with phosgene and an exchange reaction of glycerol with a dialkyl carbonate are known as conventional methods for synthesizing glycerol carbonate from glycerol. Another method for synthesizing glycerol carbonate consisting in reacting glycerol with carbon monoxide and oxygen at a high pressure is described in U.S. Pat. No. 5,359,094. More recently a new way has been proposed based on the reaction of glycerol with urea according to the global reaction as follows:

In practice this reaction is the result of two steps according to the following mechanism:

Both reactions are reversible. It means that in order to reach the total conversion of reagents into glycerol carbonate, the equilibrium of the reaction has to be shifted towards the right. The classical solution is the extraction of NH3 from the reaction medium under vacuum.
This kind of process is described in recent patents: EP 0 955 298 (ONIDOL) and 1 156 042 (KAO) and in addition in the article of J-W. Yoo and Z. Mouloungui <<Catalytic carbonylation of glycerin by urea in presence of zinc mesoporous system for the synthesis of glycerol carbonate>> in Studies in Surface Science and Catalysis 146 pages 757-760 Park et al (Editors) 2003.
These documents describe the operating conditions of the process.
According to EP 0 955 298, the process is conducted at a temperature comprised between 90° C. and 220° C. and at a pressure comprised between 1 and 20 kPa, in presence of a catalyst comprising Lewis acids sites with their anionic counterpart provided by heteroatoms. Examples of said catalysts are metallic or organometallic sulfates, such as MnSO4, ZnSO4, MgSO4, FeSO4 or hydrate of paratoluene zinc sulfate for example which are possibly associated with a support.
According to EP 1 156 042 the reaction is generally conducted in the presence of a catalyst such as a metal oxide (zinc oxide) and preferably carried out in the presence of a dehydrating agent such as anhydrous magnesium sulfate, anhydrous sodium sulfate, anhydrous calcium sulfate or a molecular sieve, in order to ensure the quality of glycerol, at a temperature comprised between 100° C. and 140° C. and at a reduced pressure comprised between 13.3 and 101 kPa.
The here-above cited article concerns the works carried out by the research team having previously leaded to the filing of EP 0 955 298. The article is dedicated to various catalyst systems using Zn as active element. These systems are heterogeneous or homogeneous. For heterogeneous systems Zinc (ZnSO4) is in the form of a mineral salt or associated with an acidic resin or on an aluminosilicate. In the homogeneous form, Zn is present as a p-toluene-sulphonate. The molar yields in glycerol carbonate reach about 80%. The experiments are conducted at a temperature of 130-150° C. under a pressure of 4 kPa. The processes described in these patents present some drawbacks. The used reagents, glycerol and urea, lead to a viscous mixture which has to be vigorously stirred in order to obtain a correct contact between the reagents and the catalyst and to provide an efficient mass transfer for ammonia removal. Moreover, such slurry does not allow an easy recovery of glycerol carbonate and recycling of the catalyst. This contact will be better when the catalyst is under homogeneous form. However in that case the separation of the catalyst from the produced glycerol carbonate contained within the reaction medium is rather problematic. Such a process carried out in batch mode cannot be implemented economically on an industrial scale considering in addition the reduced pressure conditions required to remove ammonia.